The ability to perform and achieve high speed transmissions of digital data has become expected in today's computing environment. In most cases, the transmission of digital data over longer distances is accomplished by sending the data in a high-speed serial format (i.e., one single bit after another) over a communication link designed to handle computer communications. In this fashion, data can be transferred from one computer system to another, even if the computer systems are geographically remote.
In order for high-speed serial transmission to occur, the digital data signal from inside the computer must be transformed from the parallel format into a serial format prior to transmission of the data over the serial communication link. This transformation is generally accomplished by processing the computer's internal data signal through a piece of computer equipment known as a serial link transmitter or “serializer.” The function of the serializer is to receive a parallel data stream as input and, by manipulating the parallel data stream, output a serial form of the data capable of high-speed transmission over a suitable communication link. Once the serialized data has arrived at the desired destination, a piece of computer equipment known as a “deserializer” is employed to convert the incoming data from the serial format to a parallel format for use within the destination computer system.
For high speed serializer/deserializer (HSS) transmitters, the ability to adjust the transmitter amplitude is a desirable feature. In principle, the amount of current steered to the output of a differential transmitter need only be controlled to provide an adjustment to the transmitter amplitude. However, design of the transmitter must take into consideration a maximum current situation and size the transmitter devices accordingly to handle the maximum current case. Unfortunately, such a simple approach causes power to be wasted in lower amplitude situations.
Accordingly, a need exists for a serial link transmitter design that provides power savings while maintaining adjustability in transmitter amplitude. The present invention addresses such a need.